Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 344 AM EST Fri Jan 13 2023 Valid 12Z Fri Jan 13 2023 - 12Z Mon Jan 16 2023 ...Pacific Northwest... Days 1-3... Upper troughing over the eastern North Pacific will continue to send fairly weak shortwaves into WA/OR D1 and a more defined upper low, though weakening as it moves ashore, D2 into D3. With snow levels above 5000ft D1 into D2, appreciable accumulations will be above most pass levels. The highest peaks may see well over a foot of snow D1 with light amounts D2. By D3, approaching cold-core upper low will help to lower snow levels below 4000ft amid modest QPF. This will bring some accumulation to pass level. WPC probabilities of at least 4 inches of snow D3 are generally low (<40%) in the central/northern WA Cascades (as well as farther east into central ID) and above about 3000ft. ...California/Great Basin/Four Corners region... Days 1-3... Shortwave just off the CA coast early D1 will weaken and lift northeastward, bringing a weak Atmospheric River (AR) event through the region, focused on the northern CA ranges (Klamath/Shasta-Siskiyous/Trinity) into the northern Sierra. Relatively mild SW flow will favor higher snow levels around 6000ft, slowly lowering into D2. By then, a more robust shortwave or closed low at the left exit region of a 170kt jet coupled with a trailing vort max sliding into the Desert Southwest will bring a stronger AR moisture surge (PW anomalies +2 sigma) into the Sierra and points east for D2-3. Cold front will lower snow levels as the AR surge passes through, with snow levels falling below 5000ft into D3. Several feet of snow are likely through the period over the higher terrain of the Sierra, with appreciable snow into the southern CA mountains (San Gabriel and San Bernadino) as well. From the Great Basin into the Rockies, AR will lose a bit of its punch but will still yield PW anomalies around +1 sigma through much of the Four Corners region and closer to +2 sigma over the Mogollon Rim. Height falls, approaching/extended jet across the region, frontal convergence, and upslope into the terrain will drive snowfall well over 6 inches in the terrain over Utah (Wasatch/Uintas but especially the southern UT ranges near the best dynamics) and into northern AZ into the White Mountains. As the system moves eastward into D3, focus will b into southwestern CO (San Juans) which will capitalize on favorable upslope. Over a foot of snow is likely in the higher terrain. Back into NorCal, another system will push into the region with more mountain snow above 4000ft for the northern ranges into the Sierra, continuing into D4. ...Great Lakes into the Northeast... Days 1-3... Upper trough with multiple embedded shortwaves over the eastern 1/3rd of the CONUS will continue eastward, but split off its southern end into a closed low into the Piedmont by early Saturday. A surface low will move through northern New England early D1, with a mild surge of warmer air on its east side up and over a sub-freezing boundary layer across far northern Maine. WAA-driven snow will accumulate several inches especially closer to the Canadian border where WPC probabilities are highest (over 70%), with mixed ptype just to the south. There, some sleet and freezing rain may accrete to a few tenths (sleet) and near a tenth of an inch (ice) where cold surface air is most prevalent. As the low moves out of the region, wrap-around flow will turn most precip to snow except for closer to the coast in eastern/southeastern New England as precip ends. Northerly flow off Lakes Erie/Ontario will continue some multi-band lake snows that may accumulate a few more inches before ending early Saturday. Though D2-3 show low probs for snowfall, that southern portion of the larger upper trough will support a coastal low just offshore that will bring some snow (or rain/snow over Cape Cod) to eastern/southeastern New England. ...Southern and Central Appalachians... Day 1... Post-FROPA, CAA will drive temperatures below freezing over the higher elevations from WV southward into the northern GA mountains D1. With a long-fetch modest moisture source back to the Great Lakes, upslope NW flow will focus into the Smokies along the TN/NC border with significant snowfall. Snowfall totals may approach 2 feet along the border. Over WV (and parts of far SW VA into SE KY), less robust upslope will still yield several inches of snow -- WPC probabilities of at least 4 inches are greater than 50% around Snowshoe. Fracasso ...Key Messages for Mid-January Atmospheric Rivers... -- Heavy Precipitation Spreads Inland Friday An atmospheric river will impact Northern California and the Central California coast on Friday. This will be followed by a second, more widespread system that will bring heavy precipitation, including heavy mountain snow, across much of California on Saturday and Saturday night. -- Dangerous Mountain Travel Friday through Saturday Heavy mountain snow and strong winds will lead to blowing snow at times, creating dangerous to near impossible travel above 5000 ft in the mountains and passes of Northern and Central California. -- Additional Flooding is Expected With saturated conditions already in place, limited flooding impacts are expected across Northern and Central California through next week. Limited flooding impacts are expected in portions of south-central and Southern California this weekend into early next week. -- Additional Atmospheric Rivers into Next Week Another strong atmospheric river is expected to impact the region late Sunday into Monday, bringing additional rounds of heavy precipitation including heavy mountain snow.